Allowance for Loan Losses - Age Analysis of Past Due Loan Delinquencies (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	$ 5,687,405	$ 5,260,445	$ 4,757,732
Loans in forbearance	714,516	577,164	468,402
TDR loans in repayment and percentage of each status:
Loans current	16,315,651	14,289,705	11,911,128
Total TDR loans in repayment	$ 16,787,670	$ 14,666,856	$ 12,206,033
Loans current, in percentage	97.20%	97.40%	97.60%
Loans in repayment, in percentage	100.00%	100.00%	100.00%
Total TDR loans, gross	$ 23,189,591	$ 20,504,465	$ 17,432,167
Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 288,051	$ 231,216	$ 179,002
Loans delinquent (in percentage)	1.70%	1.60%	1.50%
Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 121,302	$ 95,105	$ 78,292
Loans delinquent (in percentage)	0.70%	0.70%	0.60%
Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 62,666	$ 50,830	$ 37,611
Loans delinquent (in percentage)	0.40%	0.30%	0.30%
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	$ 87,749	$ 69,212
Loans in forbearance	99,054	69,796
TDR loans in repayment and percentage of each status:
Loans current	1,230,954	994,411
Total TDR loans in repayment	$ 1,395,163	$ 1,118,848
Loans current, in percentage	88.20%	88.90%
Loans in repayment, in percentage	100.00%	100.00%
Total TDR loans, gross	$ 1,581,966	$ 1,257,856
Troubled Debt Restructured Loans | Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 85,555	$ 63,074
Loans delinquent (in percentage)	6.10%	5.60%
Troubled Debt Restructured Loans | Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 49,626	$ 36,804
Loans delinquent (in percentage)	3.60%	3.30%
Troubled Debt Restructured Loans | Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 29,028	$ 24,559
Loans delinquent (in percentage)	2.10%	2.20%